1. Field of the Invention
The present invention relates to an ultrasonic motor, and in particular, relates to an ultrasonic motor whose service life is prolonged by reducing frictional wear between a rotor and a stator.
2. Description of the Prior Art
An ultrasonic motor includes a stator having a plurality of circumferentially arranged piezoelectric elements and a rotatable rotor in the form of an annular plate or a circular plater which is in press contact with the stator at a predetermined pressure. The rotor is provided with an abutment surface, and the stator is provided with corresponding abutment surfaces on a comb-tooth body in the peripheral direction thereof. When a high-frequency voltage is applied to the piezoelectric elements of the stator, the piezoelectric elements vibrate. The vibration is amplified in the circumferential direction by comb teeth integral with the piezoelectric elements and causes a progressive-wave motion of the comb teeth in the circumferential direction. As a result, the rotor which is in friction-contact with the piezoelectric elements rotates about an axis thereof. To this end, it is necessary to provide a mechanism to press the rotor against the stator, and hence, ultrasonic motors using a spring as a pressure source have been proposed in the prior art.
In the ultrasonic motor disclosed in Japanese Unexamined Patent Publication No. 2000-60154, a circular disc portion surrounded by the peripheral wall of the rotor, which is in the form of a short cylinder, defines a thin disc spring which provides a spring force to press the peripheral wall portion of the rotor against the comb-tooth body of the stator. In this structure, no coil spring is necessary, and thus a thinner ultrasonic motor can be achieved.
In this known ultrasonic motor, the width of an abutment surface of a peripheral wall portion of the rotor in the radial direction is smaller than the width of corresponding abutment surfaces of the comb-tooth body of the stator in the radial direction. The peripheral wall portion of the rotor is provided with an annular projection. The annular projection makes the width of the abutment surface of the rotor smaller than the width of the abutment surface of the stator so as to determine the frictional force by the width of the abutment surface of the rotor whereby the frictional force produced at the abutment surfaces of the rotor and the stator can be set to a desired value. The annular projection ensures that if the position of the rotor is deviated from the stator in the radial direction, due to parts tolerance or assembly tolerance when the rotor is assembled with the stator, the entire abutment surface of the rotor can be pressed against the abutment surface of the stator.
In this conventional ultrasonic motor, only at a part of each abutment surface of the comb teeth in the radial direction (width direction) are continuously pressed onto the abutment surface of the annular projection of the rotor. Consequently, the abutment surfaces of the comb teeth are partly worn only at the portion thereof that is always in press contact with the abutment surface of the annular projection. As a result, a depression or groove due to partial wearing is produced in the abutment surfaces of the comb teeth. The depressions or grooves cause an abnormal press-contact between the abutment surfaces of the rotor and the stator, so that an abnormal rotation such as an irregular or eccentric rotation of the rotor occurs due to an irregular frictional force. This reduces the service life of the ultrasonic motor.